1. Field of the Invention
This invention relates to systems for electrowinning an element from a feedstock compound. In particular this invention relates to apparatus and methods for producing dense, high-purity elemental deposits.
2. Background Information
The implementation of silicon-based photovoltaic technology has grown significantly in recent years. Nevertheless, an economical way of producing silicon of sufficient purity for high-efficiency solar cells—at least 99.9999% pure—has remained somewhat elusive. Solar-grade silicon is conventionally obtained by first reducing silicon dioxide carbothermically, yielding metallurgical-grade silicon, which is on the order of 98% pure. The metallurgical-grade silicon is then converted to a volatile silicon compound that may be readily purified by distillation, for example silane, tetrachlorosilane or trichlorosilane. The silicon is recovered from the purified volatile silicon compound by exposing it to solid-phase silicon substrates at high temperature, provoking decomposition of the compound with deposition of high-purity silicon onto the substrate. The deposited silicon is better than solar grade, typically greater than 99.9999%. However, this purification sequence is energy intensive, multiplying the energy needed for fundamental reduction by several powers of ten. There is, accordingly, a need for a more cost-effective way to produce silicon of optimal purity for solar applications.